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The major contaminant in biogas is H2S which is both poisonous and corrosive, and causes significant damage to piping, equipment and instrumentation. The concentration of various components of biogas has an impact on its ultimate end use. While boilers can withstand concentrations of H2S up to 1000 ppm, and relatively low pressures, internal combustion engines operate best when H2S is maintained below 100 ppm. The commonly used methods for hydrogen sulphide removal from biogas are internal to the anaerobic digestion process – air/oxygen dosing to digester biogas and iron chloride dosing to digester slurry.

Biological Desulphurization

Biological desulphurization of biogas can be performed by using micro-organisms. Most of the sulphide oxidising micro-organisms belong to the family of Thiobacillus. For the microbiological oxidation of sulphide it is essential to add stoichiometric amounts of oxygen to the biogas. Depending on the concentration of hydrogen sulphide this corresponds to 2 to 6 % air in biogas.

The simplest method of desulphurization is the addition of oxygen or air directly into the digester or in a storage tank serving at the same time as gas holder. Thiobacilli are ubiquitous and thus systems do not require inoculation. They grow on the surface of the digestate, which offers the necessary micro-aerophilic surface and at the same time the necessary nutrients. They form yellow clusters of sulphur. Depending on the temperature, the reaction time, the amount and place of the air added the hydrogen sulphide concentration can be reduced by 95 % to less than 50 ppm.

Measures of safety have to be taken to avoid overdosing of air in case of pump failures. Biogas in air is explosive in the range of 6 to 12 %, depending on the methane content). In steel digesters without rust protection there is a small risk of corrosion at the gas/liquid interface.

Iron Chloride Dosing

Iron chloride can be fed directly to the digester slurry or to the feed substrate in a pre-storage tank. Iron chloride then reacts with produced hydrogen sulphide and form iron sulphide salt (particles). This method is extremely effective in reducing high hydrogen sulphide levels but less effective in attaining a low and stable level of hydrogen sulphide in the range of vehicle fuel demands.

In this respect the method with iron chloride dosing to digester slurry can only be regarded as a partial removal process in order to avoid corrosion in the rest of the upgrading process equipment. The method need to be complemented with a final removal down to about 10 ppm.

The investment cost for such a removal process is limited since the only investment needed is a storage tank for iron chloride solution and a dosing pump. On the other hand the operational cost will be high due to the prime cost for iron chloride.

Biogas that has been upgraded by removing hydrogen sulphide, carbon dioxide and moisture is known as biomethane. Biomethane is less corrosive than biogas, apart from being more valuable as a vehicle fuel. The typical composition of raw biogas does not meet the minimum CNG fuel specifications. In particular, the CO2 and sulfur content in raw biogas is too high for it to be used as vehicle fuel without additional processing.

Liquified Biomethane

Biomethane can be liquefied, creating a product known as liquefied biomethane (LBM). Biomethane is stored for future use, usually either as liquefied biomethane or compressed biomethane (CBM) or since its production typically exceeds immediate on-site demand.

Two of the main advantages of LBM are that it can be transported relatively easily and it can be dispensed to either LNG vehicles or CNG vehicles. Liquid biomethane is transported in the same manner as LNG, that is, via insulated tanker trucks designed for transportation of cryogenic liquids.

Compressed Biomethane

Biomethane can be stored as CBM to save space. The gas is stored in steel cylinders such as those typically used for storage of other commercial gases. Storage facilities must be adequately fitted with safety devices such as rupture disks and pressure relief valves.

The cost of compressing gas to high pressures between 2,000 and 5,000 psi is much greater than the cost of compressing gas for medium-pressure storage. Because of these high costs, the biogas is typically upgraded to biomethane prior to compression.

Applications of Biomethane

The utilization of biomethane as a source of energy is a crucial step toward a sustainable energy supply. Biomethane is more flexible in its application than other renewable sources of energy. Its ability to be injected directly into the existing natural gas grid allows for energy-efficient and cost-effective transport. This allows gas grid operators to enable consumers to make an easy transition to a renewable source of gas. The diverse, flexible spectrum of applications in the areas of electricity generation, heat provision, and mobility creates a broad base of potential customers.

Biomethane can be used to generate electricity and heating from within smaller decentralized, or large centrally-located combined heat and power plants. It can be used by heating systems with a highly efficient fuel value, and employed as a regenerative power source in gas-powered vehicles.

Biomethane to Grid

Biogas can be upgraded to biomethane and injected into the natural gas grid to substitute natural gas or can be compressed and fuelled via a pumping station at the place of production. Biomethane can be injected and distributed through the natural gas grid, after it has been compressed to the pipeline pressure. In many EU countries, the access to the gas grid is guaranteed for all biogas suppliers.

One important advantage of using gas grid for biomethane distribution is that the grid connects the production site of biomethane, which is usually in rural areas, with more densely populated areas. This enables the gas to reach new customers. Injected biomethane can be used at any ratio with natural gas as vehicle fuel.

Biomethane is more flexible in its application than other renewable sources of energy.

The main barriers for biomethane injection are the high costs of upgrading and grid connection. Grid injection is also limited by location of suitable biomethane production and upgrading sites, which have to be close to the natural gas grid.

Several European nations have introduced standards (certification systems) for injecting biogas into the natural gas grid. The standards, prescribing the limits for components like sulphur, oxygen, particles and water dew point, have the aim of avoiding contamination of the gas grid or the end users. In Europe, biogas feed plants are in operation in Sweden, Germany, Austria, the Netherlands, Switzerland and France.